1. Field of the Invention
This invention relates to improvements in the art of reaction injection molding (RIM) of reactive liquid resin mixes in the production of foamed or unfoamed plastic products, and more especially the incorporation into such plastic products of a variety of particulates, either fibrous, granular, or the like, which serve as reinforcement of the finished product to meet requirements of greater strength, better insulative or lower expansion properties, or which serve other functions such as coloring, fire retarding, etc.
2. Brief Description of the Prior Art
In the typical RIM system of producing plastic products, two or more liquid polymer precursors of a reactive mix are individually stored in large tanks and are separately pumped to a mixing head where they are injected under high pressure through orifices in the head to produce impinging streams within a mixing chamber of the head. Admission of the components to the mixing chamber is controlled by a plunger which reciprocates between an extended position in which the plunger fills the mixing chamber and blocks admission of the components through the injection ports or orifices, and a retracted position in which the plunger is withdrawn from the mixing chamber to allow admission of the components through their respective orifices. This admission occurs in the form of high velocity streams as mentioned above and which impinge within the mixing chamber to effect intimate mixing and polymerizing reaction. The mixed components in the chamber are then expelled into a suitable mold through a discharge port of the mixing head on return of the plunger to its original position. Such return simultaneously shuts off further admission of components to the mixing chamber and diverts the individual components back to their respective storage tanks through respective recycle conduits. A typical RIM mixing head is disclosed in U.S. Pat. No. 4,082,512 to which reference is made for further details of a known arrangement.
Examples of resin compositions utilized in RIM production include various polymerizable materials; e.g., polyurethane, polyester, epoxy, phenol-aldehyde and urea-aldehyde resins, obtainable by commercially known methods of manufacture. For example, polyurethane resins are commonly prepared by polymerizing polyols and polyisocyanates in combination with various catalyzing, foaming, plasticizing, etc., ingredients. Reference is made to U.S. Pat. Nos. 4,055,548 and 4,112,014 for typical examples of various compositions useful in the process. There is an enormous number of available resin compositions which can be used, the selection being dependent upon ultimate desired chemical and physical characteristics of the foamed product.
For purposes of discussion here, the term resin composition is used in a generic sense to mean polymerizable plastic mixes, whether of foamed or unfoamed type. The term is not here used to designate only one component (i.e. polyol), as it is sometimes employed by production personnel.
It has been proposed heretofore to incorporate various particulate materials into reactive liquid plastic mixes produced by RIM techniques in order to modify the physical properties of the articles molded of such plastic mixes. The particulate additions mentioned include glass fiber for the purpose of increasing the strength of the molded product, and also to better match the coefficient of expansion of the molded product to that of a base (e.g. metal) part with which it may be associated. Specific examples of the latter are found in foamed and unfoamed plastic automobile parts. Reaction injection molding of plastic is often a particularly satisfactory method of production, owing to the fact that such system enables viscous, fast-setting, mixes to be utilized, producing consequent increase in production rates and better physical properties. The incorporation of particulate matter in such mixes, however, causes problems. A typical method of producing fiber reinforced plastic products is disclosed, for example, in U.S. Pat. No. 4,073,840. According to that patent, a slurry is formed of glass fibers and one or more of the resin components. The slurry is agitated to maintain the fiber in suspension, and the slurry is then pumped to a mixing head where it is mixed with the complementary resin components incorporating blowing and/or curing agents, etc., required in producing the moldable resin mix. Temperature and viscosity of the components have a large effect on the curing time and the ultimate physical properties of the product. For this reason, the components are continuously recirculated in unmixed condition between their respective storage reservoirs and the mixing head, in order to continually pass them through heat exchangers and thereby maintain optimum temperature conditions. In this way the development, between molding "shots", of off-temperature increments of the several components in the passages in and adjacent to the mixing head is minimized.
The incorporation of fiber or other particulate matter in a resin component as proposed in the foregoing patent involves incorporating the particulate directly in the main supply of one of the components. The system is accordingly subject to the disadvantage, in case it is desired to change the resin mix formulation, of having to remove all trace of the previous particulate-carrying component from the storage tank, feed and return lines, etc. Another and even more troublesome disadvantage of the prior system arises because of abrasive action of the fiber-bearing component, under the high RIM operating pressures involved, upon passing through the internal passages of the mixing head. At the usual component operating pressures of up to 2600 psi used in RIM production, this abrasion becomes excessive, especially as the percent of fiber incorporated is raised to the levels frequently desired. The usual RIM mixing head employs a control plunger which is very carefully machined to extremely close tolerances to provide a lapped, leak-proof, sliding fit in a similarly carefully machined cylinder defining the mixing chamber. The plunger also includes axial recirculation grooves to allow for return (in separate, unmixed condition) of the components during non-molding cycles of operation. The plunger and its grooves are therefore particularly susceptible to abrasive action of particulate matter in the liquid components to be mixed, and expensive maintenance and repair are encountered. The prior art has nevertheless put up with these problems in order to meet demand for molded resin products having the improved properties which such particulate incorporation provides.